Conventionally, an electrical circuit in a potentially explosive atmosphere is housed in a enclosure comprising an explosion protected container which has a removable lid. The container and its lid usually comprise machined metal castings provided with cooperating internal lips and surfaces which prevent a flame front established by arcing from the circuit, propagating between the container and lid to establish an explosive condition exteriorly of the enclosure. The enclosure thus in practice cannot be opened in an explosive atmosphere to allow adjustment of the circuit, since a risk of explosion would then occur. It is accordingly difficult to adjust the circuit whilst in use in a potentially explosive atmosphere. It has been proposed to provide an explosion protected enclosure with a potentiometer inside the enclosure, the potentiometer having an adjustment screw on the outside of the enclosure. Whilst this arrangement does permit circuit adjustments to be made, the enclosure is of a complex construction and only a limited adjustment can be made. Also, it is not readily possible to monitor the adjustment that actually has been made, nor is it possible to make the adjustment from a remote location.